In a typical radio communications network, wireless terminals, also known as mobile stations, terminals and/or user equipments, UEs, communicate via a Radio Access Network, RAN, to one or more core networks. The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g. a radio base station, RBS, or network node, which in some networks may also be called, for example, a “NodeB” or “eNodeB”. A cell is a geographical area where radio coverage is provided by the radio base station at a base station site or an antenna site in case the antenna and the radio base station are not collocated. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. Another identity identifying the cell uniquely in the whole mobile network is also broadcasted in the cell. One base station may have one or more cells. A cell may be downlink and/or uplink cell. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations.
A Universal Mobile Telecommunications System, UMTS, is a third generation mobile communication system, which evolved from the second generation, 2G, Global System for Mobile Communications, GSM. The UMTS terrestrial radio access network, UTRAN, is essentially a RAN using wideband code division multiple access, WCDMA, and/or High Speed Packet Access, HSPA, for user equipments. In a forum known as the Third Generation Partnership Project, 3GPP, telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. In some versions of the RAN as e.g. in UMTS, several base stations may be connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller, RNC, or a base station controller, BSC, which supervises and coordinates various activities of the plural base stations connected thereto. The RNCs are typically connected to one or more core networks.
Specifications for the Evolved Packet System, EPS, have been completed within the 3rd Generation Partnership Project, 3GPP, and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio Access Network, E-UTRAN, also known as the Long Term Evolution, LTE, radio access, and the Evolved Packet Core, EPC, also known as System Architecture Evolution, SAE, core network. E-UTRAN/LTE is a variant of a 3GPP radio access technology wherein the radio base station nodes are directly connected to the EPC core network rather than to RNCs. In general, in E-UTRAN/LTE the functions of a RNC are distributed between the radio base stations nodes, e.g. eNodeBs in LTE, and the core network. As such, the Radio Access Network, RAN, of an EPS has an essentially flat rather than hierarchical architecture comprising radio base station nodes without reporting to RNCs.
Device-to-Device, D2D, communication, as an underlay to cellular networks, has been proposed as a means to take advantage of the close proximity of communicating devices, i.e. UEs, and at the same time to allow these UEs to operate in a controlled interference environment. In this case, close proximity may typically refer to less than a few tens of meters, but sometimes even up to a few hundred meters. This D2D or direct mode communication may demonstrate a number of potential gains over traditional cellular communication. One of these potential gains is capacity. For example, radio resources, such as, e.g. Orthogonal Frequency-Division Multiplexing, OFDM, resource blocks, between the D2D and cellular layers may be reused, resulting in reuse gains.
In cellular or radio communications networks with integrated D2D communication capabilities, the coexistence of cellular and D2D communication should be facilitated. This is particularly important when D2D communication is introduced gradually to evolving cellular or radio communications networks when legacy UEs continue to be served for a long time after D2D capable UEs are introduced in commercial cellular or radio communications networks.
For example, when cellular downlink resources in the cellular or radio communications networks are used for D2D communication, the impact of the D2D activities on cellular downlink transmissions must be controlled and vice versa; this in order for the D2D communications to not harmfully impact the cellular downlink transmissions, and for the cellular downlink transmissions to not harmfully impact the D2D communications. This kind of co-existence requires that both idle and active mode UEs are protected and must be guaranteed also for legacy cellular UEs that are unaware of any potential D2D communications. It is especially important to protect the various measurements that the UEs perform in both idle and active mode for the purpose of, for example, cell association, cell reselection, radio link monitoring, Radio Resource Management, RRM, and Channel State Information, CSI, estimation.
Furthermore, cellular or radio communications networks, such as, for example, a 3GPP Long Term Evolution, LTE, network, are normally deployed and operated using different configurations and setups. This may include support of different LTE Transmission Modes, TMs, such as, for example, TM4, TM9, and TM10, which require dissimilar set of Reference Signals, RS, for proper operation. Here, another aspect which needs to be considered is whether or not neighboring cells and/or sites are time synchronized, and may be also time-aligned. A further aspect to consider is also whether the downlink Cell specific Reference Signal, CRS, planning is shifted or un-shifted.
D2D Communications in the Cellular Spectrum
Allowing D2D communications in the cellular spectrum of cellular or radio communications networks is a means of increasing the spectrum utilization. This is because a pair of UEs communicating in a D2D mode may then reuse the cellular spectrum resources, such as, for example, Physical Resource Blocks, PRBs, of a LTE network.
When D2D communications use the downlink, DL, resources of a cellular or radio communications network, existing RRM techniques may protect the PRBs used for DL cellular transmissions of user data. In this way, user specific RSs, such as, e.g. DM-RSs used for demodulation and transmitted within the same PRBs as the user data, are automatically protected as well. These techniques are possible since in cellular integrated D2D communications, i.e. in cellular or radio communications networks with integrated D2D communication capabilities, the network node schedules and/or grants access to resources used for both D2D and cellular transmissions. Similarly, the D2D communications may be protected from cellular downlink data transmissions, e.g. data transmitted on the Physical Downlink Shared CHannel, PDSCH, by means of the network node scheduling D2D and cellular data transmissions on orthogonal resources or PRBs.
To protect cellular downlink RS that are mandatory, such as, e.g. CRS, CSI-RS, and CSI-Interference Measurement, CSI-IM, is more challenging as these mandatory signals are regularly transmitted over the entire frequency band. Similarly, existing RRM techniques do not automatically protect the D2D communication from the cellular downlink reference signals. It may be noted that CRS transmission is mandatory in all LTE networks, whereas CSI-RS and CSI-IM are mandated only for certain TMs, such as, e.g. TM9 and TM10.
RS in Cellular or Radio Communications Networks
In virtually all cellular or radio communications networks, downlink pilots or RSs of predefined and known characteristics, are regularly transmitted in the downlink by, e.g. Access Points, APs, or network nodes. The downlink pilots or RSs are used, e.g. measured, by both idle and active UEs for the purpose of, for example, mobility measurements, cell association, and as reference for CSI estimation and data demodulation. In the case of LTE Release 8 network, some of the RSs are called Cell specific Reference Signals, CRSs. CRSs have a predefined pattern that cover the entire frequency band and are transmitted four times per millisecond, i.e. for two antenna ports.
FIGS. 1-2 show examples of the CRS arrangements in such a LTE Release 8 network. In more detail, FIG. 1 shows a schematic illustration of a PRB depicting a cell specific antenna configuration for a one-antenna port, whereas FIG. 2 shows a schematic illustration of a PRB depicting a cell specific antenna configuration for a two-by-two-antenna port. Here, it may be noted that a PRB is composed by 7 OFDM symbols in the time domain, and 12 subcarriers, i.e. 180 kHz, in the frequency domain, thus comprising 84 so called Resource Elements, REs, in the Orthogonal Frequency-Division Multiplexing, OFDM, time-frequency grid. Some of the REs are reserved to carry RS, such as, the REs marked R0 in FIG. 1 for one antenna port and the REs marked with an “x” or blackened in FIG. 2 for two antenna ports. Hereinafter, the set of time-frequency positions or indices of the REs used for RSs may be referred to as a RS pattern.
Other examples are the CSI-RSs introduced in the DL of LTE Release 10 networks which serve the purpose of helping the UE to estimate the CSI for multiple cells rather than just the serving cell, and CSI-IMs introduced in the DL of LTE Release 11 networks which are used for inter-cell interference estimation. Other cellular and wireless radio communication technologies, such as, e.g. Wide band Code Division Multiple Access, WCDMA, High Speed Packet Access, HSPA, WiMax, etc., normally provide a similar type of pilot or reference signals to support e.g. mobility measurements, channel state dependent algorithms, and/or demodulation of control and data information.
Furthermore, cellular or radio communications networks may employ different strategies for the arrangements of RS in the time-, frequency-, code- and antenna port domains. For example, in OFDM, RS may be arranged according to the so called block type, comb type or some other RS patterns, such as, e.g. the RS pattern shown in FIG. 1. The exact pattern of the RSs in time and frequency may be optimized for different objectives, and it also affects how the receiver of the RS, e.g. a cellular UE, may use time and/or frequency domain interpolations to estimate the actual CSI for demodulation or other purposes. For example, a UE may weigh in RS measurements from the past or in other frequencies than at which the actual CSI is needed at a given point in time.
From the discussion above, it may be concluded that existing solutions in a cellular or radio communications network do not facilitate a safe introduction of D2D communication in DL cellular resources that are also used for cellular communication by legacy UEs.